Flat panel images are often used in radiotherapy and other applications in order to derive an image from ionising radiation that has (for example) passed through a patient. Diagnostic images are generally obtained from the use of kilovoltage (kV) radiation and can be used either as two dimensional images or can be used as one of a number of such two dimensional images in order to create three dimensional representations via computed tomography (CT). Therapeutic radiation tends to be in the megavoltage (MV) range and can also be used to derive a portal image. This is an image of the therapeutic radiation after it has passed through the patient; generally the image is of a low quality with poor contrast. Anatomical features are however apparent in the image and can be used to check (for example) that the patient is correctly positioned.
MV radiation produces an image with inherently low contrast and therefore it is important that no artefacts are present in the image to obscure the anatomy. The MV source typically operates in a pulsed manner at a duty cycle of approximately 1 in 1000 and therefore there is ample opportunity between pulses to obtain data from a few rows of the flat panel imager.
FIG. 1 shows the typical structure of a flat panel imager 10. An upper layer 12 consists of a scintillator under the application of x-rays 14. The light 16 thus produced impinges on an array 18 of photodiodes and transistors which are disposed in a layer immediately beneath the upper layer 12. The array 18 is divided into individual pixels, each of which is associated with a single photodiode. The light impinges on the photodiode in the array and creates an electronic signal which is gated appropriately by the transistor. The electronic signal 20 thus produced is extracted from the flat panel array via read-out electronics 22 to form a digital data stream 24 that is used to construct the image.